Helical scan VTR with deflectable head

ABSTRACT

A tracking control system used in video reproducing apparatus to keep the signal head on the parallel recorded tracks in spite of speed deviations of the recording medium has a bi-morph leaf to support the head. The bi-morph leaf includes piezo-electric material to which a control voltage can be applied to deform the piezo-electric material to the extent necessary to deflect the leaf in the proper direction and to the proper extent necessary to keep the head mounted on the leaf alignment with the correct track section. When the leaf reaches its maximum permissible deflection, the control voltage is reversed to shift the head in the opposite direction. In so doing the head either skips a track section, if the recording medium is moving too fast, or repeats playback of a section, if the medium is moving too slowly. If the apparatus is reproducing magnetically recorded signals by means of two heads mounted on separate bi-morph leaves to play back alternate track sections, the control voltages can be applied to the leaves to shift each of the heads in the direction of its gap length during an interval when it is not being used to play back a track.

This is a continuation of application Ser. No. 635,139, filed Nov. 25,1976, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a system for eliminatingtracking errors in reproducing recorded video signals. Moreparticularly, the invention is directed to a video signal reproducingdevice using a piezo-ceramic material in the playback head support tocompensate for tracking errors.

2. The Prior Art

In video tape recorders video signals are recorded on magnetic tape in aseries of parallel magnetic tracks at a predetermined slant angle to thelongitudinal direction of the tape. When the tape is run through aplayback machine to reproduce the signals, a tracking servo system isemployed to make the magnetic head trace, or scan, the recorded trackscorrectly. With such a tracking servo system, the rotating speed of arotary head drum having recording and reproducing heads therein iscompared with the longitudinal speed of the tape to generate a signal tocontrol the rotating speed of the rotary head drum, or the longitudinalspeed of the magnetic tape, or both. Braking devices are commonly usedto control the capstan motor that determines the longitudinal speed ofthe tape and to control the rate of rotation of the rotary head drummotor. The amount of braking force that must be used makes it necessaryto provide a power amplifier to operate the brake devices. This in turn,requires a suitable power source, and the size of the video taperecorder (VTR) must, therefore, be large.

U.S. Pat. No. 3,787,616 describes means for correlating the position ofa magnetic playback head longitudinally along a track on which videosignals are recorded. The head is supported on a bi-morph leaf thatincludes piezo-electric sections responsive to control voltages to bend,or deflect, the leaf and thus move the head longitudinally with respectto the track to follow variations in the longitudinal speed of thetrack. However, the apparatus makes no provision for correction of anerror due to a continuing inaccurate speed of the tape which wouldeventually cause the bi-morph leaf to be bent to its maximum limit, nordoes it provide for correction of the tracking of skewed tracks typicalof video tape recordings.

SUMMARY OF THE INVENTION

According to the present invention, a system for reproducing a videosignal is provided which is convenient for use in a simple VTR and canapply a safe tracking servo control to correct the position of amagnetic head transverse the track and achieve accurate tracking ofskewed tracks as well as accurate control in spite of continuouslyinaccurate speed of the recording medium. Furthermore, this inventionresults in accurate tracking without controlling the speed of thereproducing motor and, therefore, without requiring large electric powerconsumption. To control the position of magnetic heads, in the presentinvention, a piezo-ceramic material such as a bi-morph leaf is employedon which a magnetic head chip is mounted. The piezo-ceramic material issupplied with a predetermined control voltage to be deflected so as tocontrol the position of the magnetic head transversely with respect tothe longitudinal direction of the track. The control voltage can producea sudden, relatively large shift in the head position once the head isat or near the position corresponding to maximum deflection of the leaf.This allows a track to be skipped if the medium is moving too fast or tobe played back twice if the medium is moving too slowly and continuessuch incorrect fast or slow movement too long.

Further, with the system of the present invention it is not necessarythat the motor for driving the rotary head drum be different from thatfor driving the capstan; one motor is enough for driving the both.

Accordingly, it is an object of the present invention to provide asystem for improved tracking control in reproducing a video signal in asimple VTR.

It is another object of the present invention to provide an improvementin the tracking servo system of a simple VTR in which the signalrecorded in a series of skewed tracks.

It is a further object of the invention to provide an improved servosystem in which a piezo-ceramic material is used as a tracking servodevice.

It is a still further object of the invention to provide a system forreproducing a video signal for use in a VTR that uses one motor fordriving both the capstan and rotary head drum thereof.

Other objects, features, and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows partially in cross-section, the head drum assembly of aVTR including a part of a head assembly for one embodiment of the systemfor reproducing a video signal according to the present invention.

FIG. 1B is a cross-sectional view taken on the line I--I in FIG. 1A.

FIG. 2 is an enlarged drawing of the head assembly of FIGS. 1A and 1Btogether with its control circuitry.

FIG. 3 is a graph of the displacement of the head assembly of FIG. 2 inresponse to the control voltage.

FIG. 4 is a diagram used for explaining the operation of the trackingcontrol of the present invention.

FIG. 5 is a block diagram of the basic tracking control circuitaccording to the invention.

FIG. 6 is a block diagram showing a practical example of the trackingcontrol circuit according to the invention.

FIGS. 7A to 7J are time charts used for explaining the operation of thetracking control circuit in FIG. 6.

FIG. 8 shows the head drum assembly of another embodiment of the presentinvention.

FIG. 9 is a block diagram of a tracking control circuit used for theembodiment in FIG. 8.

FIG. 10A to 10F are time charts for explaining the circuit in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B show a rotary head drum assembly and a tape driveassembly of a two head helical type VTR, constructed according to thepresent invention. In this VTR, the height, or position, of magneticheads for recording and reproducing a video signal, or for reproducing apreviously recorded video signal, can be changed arbitrarily.

The apparatus in FIG. 1A includes a stationary lower drum 1 and arotatable upper drum 2 supported on a rotatable shaft 3. The lower endof the shaft 3 extends through a chassis 4 of the VTR and is directlycoupled to a drum motor (not shown). The lower drum 1 is affixed to thechassis 4 by bolts 5. A cylinder shaft 6 of the lower drum 1 hasbearings 7 within it to provide rotatable support for the shaft 3. Theshaft 3 extends through a hub 8 of the upper drum 2, which is held onthe shaft 3 by a nut 9 so that the upper drum 2 is rotated by the shaft3 when the latter is driven by the motor. On the bottom surface 10 ofthe upper drum 2 are mounted two support leaves, each made of apiezo-ceramic material in a structure known as a bi-morph leaf. Only onesupport leaf 11a and its support member 12a are shown in FIG. 1A but asymmetrically located lead 11b and support member 12b are shown in FIG.1B. Each of the bi-morph leaves 11a and 11b includes two piezo-ceramicplates bonded to the upper and lower surfaces of a metal plate made of,for example, stainless steel or the like. The piezo-ceramic material maybe Pb(Zr-Ti)O₃. An electrode is formed on the free surface of each ofthe bi-morph leaves 11a and 11b, and in use, a predetermined voltage isapplied across these electrodes to give a mechanical bias thereto toprovide an initial deflection. Magnetic head chips 13a and 13b aremounted on the free ends of the bi-morph leaves 11a and 11b,respectively, and extend outwardly from recesses 14a and 14b formed inthe upper drum 2. A magnetic tape T is wrapped around the outerperipheries of the upper drum 2 and lower drum 1 over an arc of about180° and guided by tape guides 15a and 15b shown in FIG. 1B. Themagnetic tape T is gripped and moved longitudinally by the cooperationof a pinch roller 16 and a capstan 17. The capstan is rotatablysupported by a bearing 19 which is embedded in a boss 18 inserted intothe chassis 4. In the apparatus shown in FIGS. 1A and 1B, the capstan 17has attached thereto a pulley 20 at its lower end portion, contrary toprior art capstans. The pulley 20 is linked to the motor shaft 3 by abelt 22 stretched between the pulley 20 and a pulley 21 attached to themotor shaft 3, and no independent servo system is provided for thecapstan 17.

FIG. 2 shows, in greater detail, one of the bi-morph leaves 11a in FIGS.1A and 1B. FIG. 2 also shows a circuit for producing and controlling themechanically biased or deflected condition of the bi-morph leaf 11a. Aspreviously described, the bi-morph leaf 11a consists of two plates 100,101 made of piezo-ceramic material, the reinforcing plate 102 made ofstainless steel with the plates 100, 101 on its opposing surfaces, andthe electrodes 103, 104 formed on the free surfaces of the plates 100,101. Reference letters R indicate impedances, reference letter Pindicates the connection point of series impedances R, reference letterQ indicates an input terminal to which a control voltage w is supplied,and reference letters O and +E indicate negative and positive voltagesource terminals, respectively.

FIG. 3 is a graph showing the characteristic of the bi-morph leaf 11a inwhich abscissa represents the voltage applied across the electrodes 103,104 in volts, the ordinate represents the displacement ±Δh of the headchip 13a caused by the distortion of the bi-morph leaf 11a by the biasapplied thereto, and T_(P) shows one pitch length (corresponding to onefield) of the recorded track on the tape T of an ordinary VTR,respectively.

Turning back to FIG. 1A, when a video signal is recorded on the tape Tby a recording head, which could be the head 11a, the vertical positionof the head is maintained substantially constant, and a control signal(CTL pulse) that can be correlated with the tape position at thebeginning of each frame is recorded on the tape T as in the ordinarycase.

When the video signal is reproduced, the upper head drum 2 and thecapstan 17 are driven or rotated by the same motor. In this case,however, due to the sliding and friction of the tape T upon the tapetransportation, the rotational speed and the rotary phase of the headdrum 2 do not always coincide with the longitudinal speed of the tape T.As a result, the head cannot correctly trace the recorded track on thetape T.

If it is possible to displace or move the vertical position of the headat least two times by one pitch length of video tracks, the head cancorrectly trace out the tracks without controlling both the tape speedand the rotational speed of the head drum.

The relation between the signal reproducing heads and the recordedtracks will be now described with reference to FIG. 4. When thelongitudinal speed of the tape T travelling in the direction S increasessomewhat, the rate at which the control pulses CTL recorded on the tapeare reproduced increases and the heads 13a and 13b track an increasinglyhigher portion of the recorded track, gradually causing a trackingerror, or deviation. If the vertical position of heads is changed bydeflection of the bi-morph leaves 11a and 11b, which support the headsas shown in FIG. 1, in response to a tracking error signal, the headscan follow the shifting track positions to keep their proper trackingrelationship. However, if the compensation for the mis-tracking has tobe continued for a relatively long time, the bi-morph leaves eventuallyarrive at their position of maximum permissible deflection. The verticalposition of the heads cannot increase beyond that position. As a result,when the heads arrive at their maximum permissible deflection, they arelowered rapidly by two recording tracks widths. In the case of a VTRusing only one head, that head is lowered by one track width. If thetape speed is high continuously, the above operation may be repeatedperiodically. If the tape speed is lower than the correct speed, asimilar operation can be done to shift the heads in the oppositedirection to achieve compensation for mistracking.

In the case of a two-head VTR, when one head is reproducing the recordedsignal, the other head is idle. Therefore, it is easy to change theheight of the latter head by two track widths, corresponding to oneframe, in the idle interval, which is about 1/60th of a second long.

FIG. 4 is used to illustrate a condition in which the longitudinal speedof the tape T during reproduction is different from the recording speed.The head 13a is shown in a position Y that corresponds to the positionduring recording. The position Y' is the lowest position to which thehead is displaced during reproduction, and the position Y" is thehighest position to which the head is displaced during reproduction.

When the tape speed during reproduction is higher than the recordingspeed, the tracking-shift sooner or later, depending on the error inspeed, causes maximum permissible deflection to the position Y'. Thehead must then shift to the position corresponding to the next track tobe traced. This causes no loss of synchronization because there is nosudden displacement of horizontal synchronization. In other words, it issufficient that after the head at the position Y' has finished itsscanning on track O(n+1), which contains information for one field, thehead is shifted to the position Y" to scan track O(n+3), which would belocated next above track E(n+2), without scanning track O(n+2). It willbe recognised that the head 13a scans only alternate tracks.

Similarly, it is sufficient that the other head 13b, which has beendisplaced from its normal position X to the position X', where it tracestrack E(n+1), be displaced to the position X" to track track E(n+3),which would be above the location of track O(n+3). By this shift, apicture of one frame is skipped. However, in practice, there will be notrouble because only one frame recorded on tracks O(n+2) and E(n+2) isskipped.

Similarly, when the tape speed during playback is lower than therecording speed, the heads are controlled to trace the same track twice.For example, the head 13a that has been displaced to the position Y" isshifted to the position Y' to trace the track O(n+2) twice, and thenhead 13b is shifted from the position X" to the position X' to trace thetrack E(n+2) twice.

FIG. 5 shows a circuit for achieving the tracking servo operation of thehead assembly as described above. On the tape T one field of the videosignal is recorded, for example, as one magnetically recorded track, andone control pulse CTL is recorded per frame for carrying out thereproduction tracking servo operation along the longitudinal directionof the tape T. The tape T is wrapped around the lower and upper drums 1and 2 over an arc of about 180° and is moved longitudinally at apredetermined speed. The reproduced signals from the heads 13a and 13bare both connected to a reproducing circuit 200 which reproduces thevideo signal and delivers it to an output terminal 201.

A magnetic head 202 is provided in contact with the tape T to reproducethe control pulses CTL. The control pulses reproduced by the head 202are fed through an amplifier 203 to a phase comparator 204. A pulsegenerator device 205 is provided in connection with the rotary shaft 3of the upper drum 2 which generates, for example, a pulse per onerevolution of the shaft 3. The pulses generated by the pulse generator205 are fed to the phase comparator 204 through an amplifier 206 to bephase-compared with the control pulse CTL from the head 202. The outputfrom the phase comparator 204 is applied through a sample-and-holdcircuit 207, which includes a smoothing circuit, and an amplifier 208 tothe piezo-ceramic elements of bi-morph leaves 11a and 11b which supportthe heads 13a and 13b, respectively. In this case, the distortiondirection of the bi-morph leaves 11a and 11b is selected such that theheads 13a and 13b are displaced in the transverse direction of themagnetic track and both heads are shifted in the same direction.

Accordingly, any shift between the movement of the tape T and the rotaryphase of the upper drum 2 causes a tracking error by the heads 13a and13b. When that occurs, the phase comparator 204 produces an output whichis applied to the bi-morph leaves 11a and 11b to deflect them. Thus, theheads 13a and 13b supported by the bi-morph leaves 11a and 11b aredisplaced to trace the video tracks, which means that the tracking servooperation is carried out.

FIG. 6 is a block diagram showing the reproducing system of theinvention in detail. Components that are the same as those in FIG. 5 aredesignated by the same reference characters. The embodiment of FIG. 6will be described with reference to the graphs in FIGS. 7A to 7J. Thecontrol pulse CTL of 30 Hz (FIG. 7E) reproduced by the head 202 isapplied through the amplifier 203 to a sawtooth wave forming circuit 300to produce an output signal f (FIG. 7F) to be applied to asample-and-hold circuit 304. The sample-and-hold circuit 304 is alsosupplied with a pulse DLa of 30 Hz (FIG. 7C) as the sampling pulse tooperate in a manner which will be described later. Pulse generators 205aand 205b are magnetically coupled to the shaft 3 of the drum 2 so thateach of the pulse generators 205a and 205b produces one pulse per onerevolution of the rotary upper drum 2. In this case, the pulsegenerators 205a and 205b are located on opposite sides of the shaft 3 asin the prior art. Further, the timing of the pulse generators 205a and205b is so selected that the pulse generator 205a produces an outputpulse during the time period within which the head 13a is notreproducing a signal, while the other pulse generator 205b produces anoutput pulse during the time period within which the head 13b is notreproducing a signal. The output pulses from the pulse generators 205aand 205b are passed through the respective amplifiers 206a and 205b to apair of delay multivibrators 301 and 302, respectively, to be delayed bya predetermined time. The delay multivibrators 301 and 302 produceoutput pulses DLa and DLb (FIG. 7C and 7D), respectively, and the delaytime of the multivibrators 301 and 302 is determined so that thevertical positions of the heads 13a and 13b, which are brought to theirrest state after they are switched, can be immediately adjusted. Forexample, the pulses obtained from the delay multivibrators 301 and 302are delayed by 5 to 10 H (H being one horizontal trace period) after theswitching of the heads. In order to avoid having any vibration of thebi-morph leaf that supports the head that is not reproducing a signal bemechanically transmitted to the other head which is reproducing asignal, the adjustment of the vertical position of the momentarilyinactive head is started rapidly but is carried out gradually within theidle interval of approximately 1/60 second. After the vibration of thebi-morph leaf supporting the idle head has stopped completely, the idlehead is again made operative.

The sample-and-hold circuit 304 samples the sawtooth wave f shown inFIG. 7F from the sawtooth wave forming circuit 300 in response to eachof the sampling pulses DLa shown in FIG. 7C and then holds the sampledvoltage. The output from the sampling hold circuit 304 is applied to alow pass filter 305, if necessary, which produces a voltage g in FIG.7C. The output voltage g from the low pass filter 305 is suppliedthrough an amplifier 208a to the bi-morph leaf 11a. The output voltage gis also supplied to a sample-and-hold circuit 306, which is suppliedwith sampling pulses DLb from the delay multivibrator 302, so that thesample-and-hold circuit 306 samples the voltage g with the pulse DLb andthen supplies the sampled voltage to a low pass filter 307, ifnecessary. The low pass filter 307 produces an output voltage h shown inFIG. 7H which is applied through an amplifier 208b to the bi-morph leaf13b. Since the voltages g and h which are fed to the bi-morph leaves 11aand 11b have the same amplitude, the deflection of the leaves 11a and11b is the same and hence the heads 13a and 13b are held atsubstantially the same vertical position. Further, since twosample-and-hold circuits 304 and 306 are provided, the respective headscan be shifted to the necessary positions during their respective idlepositions.

In practice, the bi-morph leaves 11a and 11b are supplied with aninitial bias voltage, for example E/2 (E being the maximum voltage ofthe sawtooth wave f) so that whether the control voltages, which arethen outputs of the amplifiers 208a and 208b, are greater or smallerthan E/2, the bi-morph leaves 11a and 11b can be deflected to displacethe heads 13a and 13b upwardly or downwardly.

By way of example, in FIG. 2 a voltage at the connection point P betweenthe two resistors of equal resistance R is selected as E/2 to deflectthe bi-morph leaf 11a and hence to displace the height of the head 13aby +Δh or -Δh in response to whether the voltage at the input terminal Qsupplied with the control signal w is greater or smaller than E/2.Accordingly, if it is assumed that E/2 is the center voltage of thevoltages g and h in FIGS. 7G and 7F, when the voltages g and h aresupplied to the bi-morph leaves 11a and 11b, the direction and amount ofdeflection, i.e. the vertical positions of the heads 13a and 13b can beshown by curves i and j in FIGS. 7I and 7J, respectively. In fact, theresponse of the bi-morph leaves 11a and 11b to the voltages appliedthereto are as shown by the curves i and j; the leaves bend upwardly anddownwardly during the positively and negatively sloping parts,respectively, of these curves. In FIGS. 7I and 7J, points q show whenthe direction of the deflection of the bi-morph leaves 11a and 11b ischanged.

In FIG. 6, a dotted line block 308 designates a circuit to produce abias voltage having a value E/2. This bias voltage circuit consists of abattery B having a voltage E/2 and a switch S₁ that switches accordingto the recording and reproducing modes indicated on the fixed terminalsREC and PB, respectively.

Further, in the circuit in FIG. 6, the outputs of the pulse generators205a and 205b are supplied through the amplifiers 206a and 206b to aflip-flop circuit 309 the output of which is applied to a switchingcircuit 310 for selecting which of the heads 13a and 13b is to deliverits output signal through an amplifier 311 to an output terminal 312.The output signals from the heads 13a and 13b are indicated as a and bin FIGS. 7A and 7B. In this case, the overlap periods of the outputs aand b from the heads 13a and 13b which are switched by the output fromthe flip-flop circuit 309 are selected to be approximately 10 H.

FIG. 8 shows another embodiment of the invention in which the elementscommon to those of FIG. 1B are identified by the same referencecharacters. The embodiment of FIG. 8 shows a single-head type VTR inwhich the tape T is wrapped entirely around the upper drum 2. In thisexample, the head is identified by numeral 13, the bi-morph leaf bynumeral 11, and the support member by numeral 12. These elements aresubstantially the same in construction, operation and effect as theelements 13a, 13b, 11a, 11b, 12a and 12b in FIG. 1B so that theirdescription will be omitted for the sake of brevity.

FIG. 9 shows an example of the tracking control circuit for theembodiment in FIG. 8. The circuit in FIG. 9 is substantially the same asthat of FIG. 6 with the deletion of one of the heads, for example, head13b and its associated elements. If the tape having recorded informationto be reproduced by the circuit in FIG. 9 has one track per frame asdoes the tape T in FIG. 4, the shaft 3 in FIG. 8 must rotate at a speedof 60 r.p.s. The pulse generator 205 in FIG. 9 will therefore producepulses at twice the frame repetition rate. As a result, a frequencydividing counter 313 that divides by 2 is connected between theamplifier 206 and the delay multivibrator 301 so that the sawtoothsignal produced by the generator 300 will only be sampled 30 times persecond.

FIGS. 10A to 10E show signals obtained at the respective parts of thecircuit shown in FIG. 9. That is, FIG. 10A represents an output signal cof the head 13; FIG. 10B illustrates an output pulse DLc from the delaymultivibrator 301; FIG. 10C shows the control pulses CTL; FIG. 10D showsan output signal k from the sawtooth wave forming circuit 300; and FIG.10E shows an output signal l from the low pass filter 305. FIG. 10F isnot a signal waveform but a curve which shows the displacement amountand direction of the head 13, respectively.

In the case of a single-head type VTR, there is a short interval n whereno signal appears at every field as shown in FIG. 10A. Any change in thevertical position of the head should take place within the period n.

Further, when the vertical position of a single-head VTR is shifted atthe maximum deflection point of the bi-morph leaf, it is similar to thecase of a two-head type VTR in that either one frame is skipped or istraced a second time, depending on whether the tape is moving tooquickly or too slowly.

In the above embodiments of the present invention, bi-morph element orleaf is used as the piezo-ceramic material, but a mono-morph element canbe employed as the piezo-ceramic element with the same effects.

The above description is given for the case in which tracking control isto be achieved, but it is also possible to control the distance that theheads project from the head drum, to control the arcuate distancebetween two heads on the head drum to overlap the control signal forjitter correction, and so on. In the case of correcting jitter, thebi-morph leaf, and hence the head, is displaced in the lengthwisedirection of the recorded track.

If, in the case of a two-head VTR, the bi-morph leaves are employed andare deflected to twist the heads so as to provide a predeterminedazimuth angle between the gaps of two heads, it can be easily done.

The above description is made for the case in which magnetic tape isemployed as the recording medium, but the present invention can besimilarly applied to the reproduction of signals recorded on a magneticdisc as the recording medium with concentric tracks formed thereon.

Further, in the above embodiments, the recorded signal on the recordingmedium is reproduced by a magnetic head, but the present invention canbe applied to the apparatus in which signals are recorded on therecording medium as, for example, optical or capacitance variations.

It will be apparent that many modifications and variations could beeffected by one skilled in the art without departing from the spirit orscope of the novel concepts of the present invention.

What is claimed is:
 1. Apparatus for reproducing video signals recordedon a recording medium in successive parallel tracks which are skewedrelative to a direction of movement of said medium, said apparatuscomprising:signal transducer means repeatedly scanning said recordingmedium in a direction generally along said tracks for reproducing thevideo signals from said successive tracks upon said movement of themedium; controllable support means movably supporting said signaltransducer means and being responsive to variations in a trackingcontrol signal for displacing said transducer means from a neutralposition in first and second opposed directions respectively extendingtransversely with respect to the tracks, said support means havingpredetermined limits of its displacement of said transducer means insaid first and second directions from said neutral position; and controlsignal generating means for providing said tracking control signal andbeing normally operative, upon mistracking of said transducer meansrelative to a track, to vary said tracking control signal in the sensefor causing said support means to displace said transducer means in arespective one of said directions for restoring correct tracking of saidtrack by said transducer means, said control signal generating meansincluding means operative upon displacement of said transducer means tosaid limit thereof in one of said directions for varying said trackingcontrol signal in the sense for causing said support means to displacesaid transducer means in the other of said directions substantially tothe respective limit thereof.
 2. The apparatus according to claim 1; inwhich the recorded video signals include vertical blanking periods, andsaid control signal generating means includes means to vary saidtracking control signal during said blanking periods.
 3. The apparatusaccording to claim 1; in which said transducer means includes aplurality of heads alternatively scanning said recording medium so thateach of said heads has alternating operative and inoperative periods forreproducing the video signals, said heads have respective controllablesupport means, and said control signal generating means includes meansto vary said tracking control signal applied to each of said supportmeans only during said inoperative period of the respective one of saidheads.
 4. The apparatus according to claim 2; in which, in at leastcertain of said vertical blanking periods, said tracking control signalis varied to cause stepwise movement of said signal transducer meansthrough a maximum distance equal to a whole multiple of the pitch ofsaid tracks.
 5. The apparatus according to claim 4; in which said signaltransducer means includes two signal transducer heads which alternatelyscan said successive tracks, each of said tracks has a field of saidvideo signals recorded therein, and said maximum distance of thestepwise movement is twice said pitch of the tracks.
 6. The apparatusaccording to claim 4; in which said signal transducer means includes asingle head by which said tracks are scanned in succession, each of saidtracks has a field of the video signals recorded therein, and saidmaximum distance of the stepwise movement is equal substantially to saidpitch.